Ying Qin, En-Wei Liang, Shuang-Xi Yi, Yun-Feng Liang, Lin Lin, Bin-Bin Zhang, Jin Zhang, Hou-Jun Lu, Rui-Jing Lu, Lian-Zhong Lu, Bing Zhang
The durations (T_{90}) of 315 Fermi GRBs detected with the GBM-NaI detectors (8-1000 KeV) by 2011 September are calculated using the Bayesian Block method. We compare the T_{90} distribution of the GBM-NaI sample to that observed with CGRO/BATSE (25-2000 keV), BeppoSAX/GRBM (40-700 KeV), HETE-2/FREGATE (6-80 keV), and Swift/BAT (15-150 keV). We show that the T_{90} distribution of the GBM-NaI GRB sample is bimodal, with a statistical significance level being comparable to that observed with BeppoSAX/GRBM and Swift/BAT but much lower than that observed with CGRO/BATSE. No bimodal T_{90} distribution is found in the HETE-2 GRB sample. Taking T_{90}=2 seconds as the division line between long and short GRBs, the ratio of short to long GRB numbers are 0:95, 51:557, 111:892, 39:253, and 500:1541 for the HETE-2/FREGATE, Swift/BAT, BeppoSAX/GRBM, GBM-NaI, and CGRO/BATSE GRB samples, respectively. These results suggest that the bimodal T_{90} distribution would be due to an instrumental selection effect. We investigate the energy dependence of T_{90} by measuring the T_{90} in the 8-15 KeV, 15-25 keV, 25-20 KeV, 50-100 keV, 100-350 keV, and 350-1000 keV energy bands with the GBM-NaI data. It is found that the T_{90} distributions in different energy bands are roughly consistent with the those derived from the instruments with similar corresponding energy bands. The value of T_{90} as a function of energy follows a relation T_{90}~ E^{-0.15}. Including X-ray flares, we found that the central engine activity time scale can be even much longer for some GRBs. Our results, together with the observed extended emission of Type I GRBs and significant flares in both the Type I and Type II GRBs, not only challenges the long-short GRB classification scheme, but also challenges the conventional GRB central engine models.
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http://arxiv.org/abs/1205.1188
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